This study assesses the relative efficacy and safety of azimilide 35mg daily vs. 75 mg daily vs. placebo in the treatment of atrial fibrillation/ flutter and paroxysmal supraventricular tachycardia. The primary objective is to assess the efficacy of the treatments in prolonging the tachycardia-free period. The secondary objective is to assess the effect of 35 mg and 75 mg of azimilide on heart rate during episodes of symptomatic tachycardia. An additional objective is to assess the safety of the active treatment.